Image sensors can capture an image using a rolling shutter mode or a global shutter mode. In the global shutter mode, the pixels in the image sensor capture the image at a single point in time. Each pixel in the image sensor begins and ends an integration or exposure period at the same time. In the rolling shutter mode, different lines of pixels (e.g., rows) have different exposure times as the signals are read out of the pixels line by line. Each line of pixels will start and end its exposure slightly offset in time from the other lines of pixels in the image sensor. Thus, different lines of the image are captured at slightly different points in time.
Typically, light sources such as incandescent bulbs or fluorescent tubes flicker at the same frequency as their power supply, usually 100 Hz or 120 Hz. This flicker can be captured by some or all of the pixels in an image sensor when an image is captured. The flicker can produce horizontal bands in the captured image depending on the state of the light source at the point in time that each line in the captured image was exposed. A person viewing the captured image may detect the horizontal band or bands in the image.
In some situations, an image sensor can avoid capturing flicker in an image by using exposure times that are multiples of an assumed frequency. For example, to avoid flicker caused by a 60 Hz power supply, the image sensor may choose integration times that are multiples of 1/120 of a second. In this way, each line of the image sees an integer multiple of flicker periods. But if the power supply is a 50 Hz power supply instead of a 60 Hz power supply, a horizontal band caused by a 50 Hz flicker will move predictably in the captured image. An image sensor or imaging system may be able to detect the 50 Hz flicker and responsively adjust the exposure time to avoid the 50 Hz flicker.
An image sensor, however, is not always able to detect flicker in an image when the image sensor uses exposure times that are multiples of an assumed frequency. For example, when an image sensor using a frame rate of 60 frames per second operates with exposure times that are multiples of 50 Hz, flicker produced by a 60 Hz frequency may not be detectable because a horizontal band will not move in the image. Instead, the horizontal band will appear at the same location in the captured images, making it difficult to distinguish the horizontal band from the content in the scene being imaged.